PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sun Aug 30 09:48:22 CDT 2015

The team at the LCM sent is a listing of their Kaleidoscope program. We
toggled it in and it runs great! You fiddle with the bottom two analog
input knobs to find settings that will make patterns. You can watch it
here: https://youtu.be/_KZG_kE-K-0

Two more days of debugging on the PDP-12 LINCtape controller didn't
accomplish much. We now a lot more about what is not broken, but haven't
found the intermittent problem with reading tapes.

We reran the Tape Quickie and TAPE CONTROL TEST, PART 1 and 2 to make sure
that the controller was still OK.

We tried reformatting LINCtapes on the right drive in the TU56. It
sometimes worked OK, sometimes not. It doesn't seem to matter what tape we
use, the intermittent behavior is the same. A LINCtape that formatted OK
worked fine on the Tape Data diagnostic for quite a while and then halted,
so the TC12 and the TU56 are mostly working.

We noticed that if we were searching for a tape block on the right drive
with the left drive OFF, it usually worked OK. With the left drive in LOCAL
or REMOTE the right drive had trouble finding blocks. The only change
between OFF and LOCAL & REMOTE is that the power to the motors is enabled. The
motor power supply voltages looked OK in OFF and LOCAL & REMOTE.
We swapped lots of TU56 boards between the left and right drives and a
spare TU56, but didn't find a problem yet.

Since we replaced the tape head on the right drive we speculated that we
introduced a tape head skew problem that was causing problems when reading
tapes created on other drives. The left drive was untouched, so we thought
that one might work better. The left motor for the left drive in the TU56
was sticking so much that it would not turn under its own power.
We suspected bad bearings, so that should be an easy mechanical fix. When
we removed the motors we found that the motor shafts turned freely. We
found that there is a pair of bushings and spring on the motor shaft that
take up the play and make the tape alignment more accurate.
The lubricant on bushings and shaft had dried out and was sticky, making it
difficult to rotate the shaft. Unfortunately after cleaning and reassembly
we found that the left drive would barely read a tape. Maybe this is
related to the previous problem?

Warren saw evidence on the logic analyzer traces of the TC12 that might
indicate a timing problem in the TC12 LINCtape controller. Next Friday we
will check the TC12 and TU56 timing adjustments.

Michael Thompson

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